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61.
The magmatic phase of the AD 79 eruption of Vesuvius produced alternations of fall and pyroclastic density current (PDC) deposits. A previous investigation demonstrated that the formation of several PDCs was linked with abrupt increases in the proportion of denser juvenile clasts within the eruptive column. Under the premise that juvenile clast density is controlled by vesiculation processes within the conduit, we investigate the processes responsible for these variations at or close to fragmentation levels. Pumice textures (vesicle sizes, numbers, and connectivity combined with crystal textures) from the AD 79 PDC deposits are compared to those from interbedded fall samples. Both PDC and fall deposits preserve textures that represent a full spectrum of degassing and outgassing processes, from bubble nucleation to collapse. Combining the textural and volatile (groundmass H2O) data, we derive a conduit model that satisfies all the textural and physical observations made for this phase of the eruption: lateral vesicularity/density stratifications are produced by maturing of bubble textures with superimposed localized shearing of bubble-rich magmas, which enhance outgassing of H2O. The incorporation of denser slower-moving magma from the conduit margins (??lateral magma density gradient??) is likely to be responsible for the higher abundances of dense juvenile pumice that triggered partial column collapses. We also illustrate how variations in the fragmentation depth (tapping a ??vertical magma density gradient??) can be responsible for variations in erupted clast density distributions, and potentially in the extent of degassing/outgassing. 相似文献
62.
A reappraisal of shear wave splitting parameters from Italian active volcanic areas through a semiautomatic algorithm 总被引:1,自引:0,他引:1
Shear wave splitting parameters represent a useful tool to detail the stress changes occurring in volcanic environments before
impending eruptions. In the present paper, we display the parameter estimates obtained through implementation of a semiautomatic
algorithm applied to all useful datasets of the following Italian active volcanic areas: Mt. Vesuvius, Campi Flegrei, and
Mt. Etna. Most of these datasets have been the object of several studies (Bianco et al., Annali di Geofisica, XXXXIX 2:429–443,
1996, J Volcanol Geotherm Res 82:199–218, 1998a, Geophys Res Lett 25(10):1545–1548, 1998b, Phys Chem Earth 24:977–983, 1999, J Volcanol Geotherm Res 133:229–246, 2004, Geophys J Int 167(2):959–967, 2006; Del Pezzo et al., Bull Seismol Soc Am 94(2):439–452, 2004). Applying the semiautomatic algorithm, we confirmed the results obtained in previous studies, so we do not discuss in much
detail each of our findings but give a general overview of the anisotropic features of the investigated Italian volcanoes.
In order to make a comparison among the different volcanic areas, we present our results in terms of the main direction of
the fast polarization (φ) and percentage of shear wave anisotropy (ξ). 相似文献
63.
Rosa Anna Corsaro Lucia Civetta Valeria Di Renzo Lucia Miraglia 《Bulletin of Volcanology》2009,71(7):781-793
Following the 2001 and 2002–2003 flank eruptions, activity resumed at Mt. Etna on 7 September 2004 and lasted for about 6 months.
This paper presents new petrographic, major and trace element, and Sr–Nd isotope data from sequential samples collected during
the entire 2004–2005 eruption. The progressive change of lava composition allowed defining three phases that correspond to
different processes controlling magma dynamics inside the central volcano conduits. The compositional variability of products
erupted up to 24 September is well reproduced by a fractional crystallization model that involves magma already stored at
shallow depth since the 2002–2003 eruption. The progressive mixing of this magma with a distinct new one rising within the
central conduits is clearly revealed by the composition of the products erupted from 24 September to 15 October. After 15
October, the contribution from the new magma gradually becomes predominant, and the efficiency of the mixing process ensures
the emission of homogeneous products up to the end of the eruption. Our results give insights into the complex conditions
of magma storage and evolution in the shallow plumbing system of Mt. Etna during a flank eruption. Furthermore, they confirm
that the 2004–2005 activity at Etna was triggered by regional movements of the eastern flank of the volcano. They caused the
opening of a complex fracture zone extending ESE which drained a magma stored at shallow depth since the 2002–2003 eruption.
This process favored the ascent of a different magma in the central conduits, which began to be erupted on 24 September without
any significant change in eruptive style, deformation, and seismicity until the end of eruption. 相似文献
64.
Lucia S. Herbeck Miriam Sollich Daniela Unger Marianne Holmer Tim C. Jennerjahn 《Marine pollution bulletin》2014
The impact of pond aquaculture effluents on the distribution and performance of seagrasses was examined in NE Hainan, tropical China. Samples were taken along transects in three back-reef areas with different extent of aquaculture production in their hinterland. High δ15N in seagrass leaves and epiphytes (6–9‰) similar to values in pond effluents documented aquaculture as dominant nitrogen source in the back-reefs with decreasing impact with distance from shore. Seagrass species abundance, shoot density and biomass were lower and concentrations of nutrients, chlorophyll and suspended matter were higher at nearshore sites with high and moderate pond abundance than at the control site. High epiphyte loads and low δ34S in seagrass leaves suggest temporal shading and sulphide poisoning of the nearshore seagrasses. Observed gradients in environmental parameters and seagrass performance indicate that the distance from the pond outlets and size of the adjacent pond agglomeration are major determinants of seagrass degradation. 相似文献
65.
In this work an overview of the potential rock fall source areas and propagation assessment in the Province of Potenza territory has been presented. The rock fall process is characterized by two steps: the detachment of blocks and subsequently their propagation along the slope. The adopted methodology, used for the first time in the study area, and the software Histofit and FlowR have been very useful tools for the preliminary assessment of rock fall susceptibility at a regional scale, in particular because they have required low data of the study area. Only the DEM may be sufficient together with an appropriate choice of the input parameters and algorithms, that is to say: calculation method, directions algorithm, inertial algorithm and friction loss function. The output of the model is a map of the rock fall source areas, the propagation probabilities and the propagation kinetic energy. The results show that the adopted methodology is successful for the identification of rock fall source areas at a regional scale and the propagation probability obtaining an interesting rock fall susceptibility map. 相似文献
66.
Greig A. Paterson Andrew P. Roberts Conall Mac Niocaill Adrian R. Muxworthy Lucia Gurioli José G. Viramonté Carlos Navarro Shoshana Weider 《Bulletin of Volcanology》2010,72(3):309-330
Paleomagnetic data from lithic clasts collected from Mt. St. Helens, USA, Volcán Láscar, Chile, Volcán de Colima, Mexico and
Vesuvius, Italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to
highlight the usefulness of the paleomagnetic method for determining emplacement temperatures. At Mt. St. Helens, the temperature
of the deposits (T
dep
) at three sites from the June 12, 1980 eruption was found to be ≥532°C, ≥509°C, and 510–570°C, respectively. One site emplaced
on July 22, 1980 was emplaced at ≥577°C. These new paleomagnetic temperatures are in good agreement with previously published
direct temperature measurements and paleomagnetic estimates. Lithic clasts from pyroclastic deposits from the 1993 eruption
of Láscar were fully remagnetized above the respective Curie temperatures, which yielded a minimum T
dep
of 397°C. Samples were also collected from deposits thought to be pyroclastics from the 1913, 2004 and 2005 eruptions of
Colima. At Colima, the sampled clasts were emplaced cold. This is consistent with the sampled clasts being from lahar deposits,
which are common in the area, and illustrates the usefulness of the paleomagnetic method for distinguishing different types
of deposit. T
dep
of the lower section of the lithic rich pyroclastic flow (LRPF) from the 472 A.D. deposits of Vesuvius was ~280–340°C. This
is in agreement with other, recently published paleomagnetic measurements. In contrast, the upper section of the LRPF was
emplaced at higher temperatures, with T
dep
~520°C. This temperature difference is inferred to be the result of different sources of lithic clasts between the upper
and lower sections, with the upper section containing a greater proportion of vent-derived material that was initially hot.
Our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature
estimation. 相似文献
67.
Marco Massa Francesca Pacor Lucia Luzi Dino Bindi Giuliano Milana Fabio Sabetta Antonella Gorini Sandro Marcucci 《Bulletin of Earthquake Engineering》2010,8(5):1175-1187
The Italian strong-motion database was created during a joint project between Istituto Nazionale di Geofisica e Vulcanologia
(INGV, Italian Institute for Geophysics and Vulcanology) and Dipartimento della Protezione Civile (DPC, Italian Civil Protection). The aim of the project was the collection, homogenization and distribution of strong motion data acquired in Italy in the
period 1972–2004 by different institutions, namely Ente Nazionale per l’Energia Elettrica (ENEL, Italian electricity company), Ente per le Nuove tecnologie, l’Energia e l’Ambiente (ENEA, Italian energy and environment organization) and DPC. Recently the strong-motion data relative to the 23th December 2009, Parma (Mw = 5.4 and Mw = 4.9) and to the April
2009 L’Aquila sequences (13 earthquakes with 4.1 ≤ Mw ≤ 6.3) were included in the Italian Accelerometric Archive (ITACA) database
(beta release). The database contains 7,038 waveforms from analog and digital instruments, generated by 1.019 earthquakes
with magnitude up to 6.9 and can be accessed on-line at the web site . The strong motion data are provided in the unprocessed and processed versions. This article describes the steps followed
to process the acceleration time series recorded by analogue and digital instruments. The procedures implemented involve:
baseline removal, instrumental correction, band pass filtering with acausal filters, integration of the corrected acceleration
in order to obtain velocity and displacement waveforms, computation of acceleration response spectra and strong motion parameters.
This procedure is applied to each accelerogram and it is realised to preserve the low frequency content of the records. 相似文献
68.
José Luis Arce Rodolfo Macías Armando García Palomo Lucia Capra José Luis Macías Paul Layer Hernando Rueda 《Journal of Volcanology and Geothermal Research》2008
Zempoala is an extinct Pleistocene (∼ 0.7–0.8 Ma) stratovolcano that together with La Corona volcano (∼ 0.9 Ma) forms the southern end of the Sierra de las Cruces volcanic range, Central Mexico. The volcano consists of andesitic and dacitic lava flows and domes, as well as pyroclastic and epiclastic sequences, and has had a complex history with several flank collapses. One of these collapses occurred during the late Pleistocene on the S–SE flank of the volcano and produced the Zempoala debris avalanche deposit. This collapse could have been triggered by the reactivation of two normal fault systems (E–W and NE–SW), although magmatic activity cannot be absolutely excluded. The debris avalanche traveled 60 km to the south, covers an area of 600 km2 and has a total volume of 6 km3, with a calculated Heim coefficient (H/L) of 0.03. Based on the textural characteristics of the deposit we recognized three zones: proximal, axial, and lateral distal zone. The proximal zone consists of debris avalanche blocks that develop a hummocky topography; the axial zone corresponds with the main debris avalanche deposit made of large clasts set in a sandy matrix, which transformed to a debris flow in the lateral distal portion. The deposit is heterolithologic in composition, with dacitic and andesitic fragments from the old edifice that decrease in volume as bulking of exotic clasts from the substratum increase. Several cities (Cuernavaca, Jojutla de Juárez, Alpuyeca) with associated industrial, agricultural, and tourism activities have been built on the deposit, which pose in evidence the possible impact in case of a new event with such characteristics, since the area is still tectonically active. 相似文献
69.
70.
Two groups of rocks are distinguished in the Assab volcanic range (Afar) on the basis of their petrology. The first one is a typical alkaline suite with high Li, Ba, Rb, Sr, Th and U concentrations. A second subalkaline group may be related to a parental basalt characterized by low trace-element concentrations and intermediate in character between alkali and tholeiitic basalt.U and Th concentrations of the analyzed Assab basalts, as well the reported values for the alkali and transitional basalts of the East African Rift and the Afar depression, are higher than values reported for similar basalts from oceanic and continental environments. 相似文献